The value of $\sum_{n=0}^{\infty}\left(\frac{2 i}{3}\right)^n$ is

Bulk of carbon dioxide $(CO_2)$ released from body tissues into the blood is present as

The process of nitrogen fixation by $Rhizobium$ is stimulated by the presence of which of the following?

Give the correct order of initials $T$ or $F$ for the following statements. Use $T$ if the statement is true and $F$ if it is false.
$(I)$ Sulphide ions react with $Na_2[Fe(CN)_5(NO)]$ to form a purple-coloured compound $Na_4[Fe(CN)_5(NOS)]$. In the reaction,the oxidation state of iron changes.
$(II)$ $Pt(IV)$ compounds are relatively more stable than $Ni(IV)$ compounds.
$(III)$ The welding of magnesium can be done in an atmosphere of Helium.
$(IV)$ $LiAlH_4$ on hydrolysis will give $H_2$.

Manganese is required in

The two lenses of an achromatic doublet should have: